Surge arrester

ABSTRACT

A surge arrester including a stack of a plurality of cylindrical varistor blocks that are arranged one after the other in the axial direction of the varistor blocks between an upper end electrode and a lower end electrode. Arranged around the stack are clamping members of an insulating material including at least three loops of continuously wound fiber that connect the upper end electrode to the lower end electrode as well as a bursting-protective bandage in the form of a plurality of rings wound of fiber, and a surrounding, electrically insulating, outer casing of rubber or other polymeric material. The loops are wound from glass fiber and exhibit an asymmetrical cross section.

TECHNICAL FIELD

The present invention relates to a surge arrester comprising a stack ofa plurality of cylindrical varistor blocks, which are arranged one afterthe other in the axial direction of the varistor blocks, between anupper end electrode and a lower end electrode. Arranged around the stackare clamping members of insulating material, comprising at least threeloops of continuously wound fibre, which connect the upper end electrodeto the lower end electrode, and a busting-protective bandage in the formof a plurality of rings or bands wound of fibre, and a surrounding,electrically insulating, outer casing of rubber or other polymericmaterial.

BACKGROUND ART

Surge arresters are used to protect expensive electrical equipment fromovervoltages. A common product for this purpose are cylindrical blocksof metal oxide, for example zinc oxide, so-called varistors. These havethe property that the resistance is high at low voltage but low at highvoltage. When the operating voltage is higher than the varistor is ableto resist and exhibit a high resistance, several varistor blocks areconnected in series in a stack. To carry large heavy currents through astack, a sufficient contact pressure must be achieved between theblocks.

U.S. Pat. No. 5,291,366 (CH 682 858) discloses a surge arrester with astack of zinc-oxide blocks clamped between two end electrodes with theaid of a clamping member consisting of two insulating elementsconnecting the two end electrodes.

U.S. Pat. No. 5,912,611 (SE 504 075) discloses a surge arrester with astack of zinc-oxide blocks clamped between two end electrodes with theaid of a clamping member consisting of at least three insulatingelements connecting the two end electrodes.

To achieve improved resistance to transversal mechanical influence, acentral pivot member is placed between one end electrode and the nearestzinc-oxide block in the stack.

The dimensioning of a surge arrester is critical and since its functionas protection for, for example, a transformer implies that it is tocarry a large current for a short period, the risk of breakdown cannever be completely excluded. This may occur, for example, by ionizationand electrical discharges in or around the varistor blocks which, bymeans of pressure increase caused by gas generation, may burst thecasing of the surge arrester.

For this reason, it is not suitable for the casing to be made of amaterial that may be fragmented at an internal pressure increase, butinstead to be made of rubber or a similar material. On the one hand, thecasing should be so strong that it may actively counteract that parts ofthe varistors are thrown out. On the other hand, the casing should beable to permit pressure relief by releasing generated gas withoutcompletely bursting the casing.

In U.S. Pat. No. 5,050,032 (SE 516 123), a balance has been struckbetween the above-mentioned requirements, wherein a varistor stack andcompression loops are radially surrounded by a busting-protectivebandage of insulating material provided with openings for pressurerelief. The bursting-protective bandage may consist of a plurality oftubular rings arranged at a certain axial distance between them. Thecasing, for example of rubber, is cast on so that the material alsofills up the space between the varistor stack and the rings. Thebursting-protective bandage may consist of a thermosetting resin withcontinuously wound glass or aramide fibres and will then have anessentially square shape.

By ring is to be understood, in SE 516 123 and in this application,essentially every closed curve and thus also curves which deviate from acircular shape. Experiences in practice have proved that the embodimentaccording to said SE 516 123 has several critical parameters. If therings are too far away from the varistor stack, the volume of theinsulant has to be increased, which, of course, increases the cost, butabove all it deteriorates the short-circuit performance because therubber, or corresponding material, inside the rings prevents thearrester from ventilating and a higher pressure is built up. Thisresults in a much more violent short-circuit behaviour. For this reason,the rings should be as close to the stack as possible. On the otherhand, the rings must not be in direct contact with the blocks. If thereis no gap between the rings and the block, filled with rubber or thelike, an exceedingly powerful bursting of the blocks is obtained, thewindings being torn off and pieces of the blocks being thrown out.

For natural reasons, the proposed, approximately square winding providesa considerable variation of the distance between the blocks and therings. To this is to be added the fact that the successive applicationof several turns compresses the loops and reduces the stress, provides aslack, in the innermost turns. These turns will then hang down towardsthe stack. See FIG. 2. There is a considerable risk that the “belly”reaches the stack unless winding is performed with a decreasing tensilestress.

OBJECT OF THE INVENTION

It is a first object of the present invention to provide a surgearrester for medium voltage and high voltage with a predictablebehaviour in case of a breakdown.

It is a second object of the present invention to provide a surgearrester that can be manufactured with less variation of performancethan hitherto known arresters.

It is a main object of the invention to provide a surge arrester thathas improved short-circuit performance, that has smaller volume, andthat may be manufactured in a more economic manner than according to theprior art.

SUMMARY OF THE INVENTION

The present invention relates to a surge arrester comprising a stack ofa plurality of cylindrical varistor blocks, which are arranged one afterthe other in the axial direction of the varistor blocks, between anupper end electrode and a lower end electrode. Arranged around the stackare clamping members of insulating material comprising at least threeloops of continuously wound fibre, which connect the upper end electrodeto the lower end electrode, and a busting-protective bandage in the formof a plurality of rings or bands wound of fibre, and a surrounding,electrically insulating, outer casing of rubber or other polymericmaterial.

In the surge arrester according to the invention, the loops are wound ofglass fibre and exhibit an asymmetrical cross section.

GENERAL DESCRIPTION OF THE INVENTION

The inventive concept is based on the realization that surge arrestersare sometimes loaded such that they break down and that this may eveninvolve danger to humans and nearby equipment, and on the experiencethat it has so far proved to be difficult to manufacture large series ofsurge arresters with a uniform quality, measurable performance, andpredictable breakdown behaviour.

To solve this problem, the present invention suggests building a surgearrester comprising a stack of a plurality of cylindrical varistorblocks, between an upper end electrode and a lower end electrode. Aroundthe stack there are placed clamping members of insulating material andcomprising at least three loops of continuously wound fibre, whichconnect the upper end electrode to the lower end electrode.

The loops are wound of glass fibre and exhibit an asymmetrical crosssection. By this is meant that the cross section of the two strands ofthe loops are mirror images of each other, that is, if a radial sectionis made through the surge arrester, the section cuts through each looptwice and the cut surfaces obtained are mirror images of each other butcannot, without rotation, cover each other. Thus, cut surfaces whichhave one or more symmetry axes may very well be used within the scope ofthe invention, as long as the two cut surfaces are mirror images of eachother and the respective symmetry axes are not parallel.

A bursting-protective bandage in the form of a plurality of rings orbands is arranged around the stack of varistors and clamping members.The rings or bands are suitably wound of aramide or PBO fibre with anepoxy or vinyl ester matrix.

The loops are to make contact with the stack and the bursting-protectivebandage is to make contact with the loops so that these are pressedagainst the stack.

It is important that the asymmetrical cross section of the loops is soshaped and placed that not only two corners, one on each strand, makecontact with the varistor stack as in the prior art with loops ofrectangular cross section. The asymmetrical cross section of the loopsmay suitably be adapted to increase the contact surface against thevaristor stack.

Further, the asymmetrical cross section of the loops may be adapted toshorten the free span for the rings or bands inside the loops and/or beadapted to enable the rings or bands to be wound closer to the stack.

It is also possible to adapt the asymmetrical cross section of the loopsso that the shape of the rings or the bands becomes approximatelycircular.

In a preferred embodiment, the cross section of the loops correspondsessentially to two mirror-inverted rhombs or rhomboids.

BRIEF DESCRIPTION OF THE DRAWING

The invention will now be explained in greater detail with reference tothe accompanying drawing, wherein

FIG. 1 schematically shows a surge arrester embedded into anelectrically insulating casing;

FIG. 2 schematically shows the same surge arrester prior to beingembedded into the electrically insulating casing;

FIG. 3 schematically shows an axial section through the surge arresteraccording to FIG. 2;

FIG. 4 schematically shows a radial section through the surge arresteraccording to FIG. 2;

FIG. 5 schematically shows, in a way corresponding to that in FIG. 4, aradial section through a prior art surge arrester; and

FIG. 6 is a reduced and somewhat simplified representation of FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a surge arrester 1 with an upper end electrode 11 and alower end electrode 12 and an electrically insulating casing 19.

FIG. 2 shows a corresponding surge arrester 1 without the electricallyinsulating casing. A stack 10, comprising seven cylindrical varistorblocks 10 a of circular cross section, is arranged between the upper endelectrode 11 and the lower end electrode 12. The symmetry axes of thevaristor blocks 10 a coincide. The diameter of the varistor blocks 10 ais 60 mm and their height 40 mm. Arranged around the stack 10 areclamping members 15 in the form of four loops, of which three 15 a areshown in the figure. The loops 15 a are wound of continuous glass fibreand impregnated with epoxy. The loops 15 a run around the shoulders 11 aof the upper end electrode 11 and the shoulders 12 a of the lower endelectrode 12 and clamp the end electrodes 11, 12 against the stack 10,thus creating the desired contact pressure between the varistor blocks10 a. The loops 15 a make contact with the stack 10 of varistor blocks10 a.

Outside the loops 15 a there is a bursting-protective bandage 16 in theform of seven rings 16 a, placed essentially at the centre of the heightfor the respective varistor blocks 10 a. The rings 16 a are wound ofaramide fibre in an epoxy matrix and make tight contact with the loops15 a so that these are pressed against the varistor blocks 10 a. Theheight of the rings 16 a is 20 mm and the their thickness is 5 mm.Between two adjacent rings 16 a, opposite to the contact surface betweenthe varistor blocks 10 a in question, there is an annular opening 17,about 20 mm high, to allow pressure relief.

FIG. 3 shows an axial section through the same surge arrester 1 as inFIG. 2, that is, without the electrically insulating casing. In additionto the features shown in FIG. 2, there are shown between the lower endelectrode 12 and the stack 10 a pivot washer 14 and between the upperend electrode 11 and the stack 10 a length adjustment device 13. For thesake of simplicity, the length adjustment device 13 is not shown indetail, but the task of the device is to lengthen the stack so that theclamping force in the loops 15 a really provides the desired contactpressure between the varistor blocks in the stack 10. The end electrodesare provided with threaded holes 11 b, 12 b to function as electricalconnection or make possible a series connection of two or more surgearresters 1.

FIG. 4 shows, by means of a radial section through the same surgearrester 1 as in FIG. 2, that is, without the gelectrically insulatingcasing, a section of the surge arrester 1. The section shows a varistorblock 10 a, a clamping member with four loops 15 a, and a surroundingbursting-protective bandage consisting of a ring 16 a of aramide fibrewith an epoxy matrix. The section through the loops 15 a exhibits, foreach loop 15 a, pairwise mirror-inverted rhomboids V, H.

FIG. 5 shows, in the same way as FIG. 4, by means of a radial sectionthrough a prior art surge arrester 2, a section of the surge arrester 2.The surge arrester 2 is made, for example, according to U.S. Pat. No.5,050,032 (SE 516 123 C2), where a stack of varistor blocks 20 a issurrounded by a clamping member with four loops 25 a of rectangular,symmetrical cross section, which in turn are surrounded by abursting-protective bandage in the form of rings 26 a of aramide fibre.

In FIGS. 4 and 5, the appearance of the rings 16 a and 26 a is onlyschematically shown by means of five lines, the shapes of whichintentionally are not drawn to scale. The purpose is to illuminate theparticular problems which may arise during manufacture and which thepresent invention is intended to reduce or, hopefully, completelyeliminate.

1. A surge arrester, comprising: a stack of a plurality of cylindricalvaristor blocks, which are arranged one after another in an axialdirection of the varistor blocks, an upper end electrode and a lower endelectrode, clamping members of insulating material comprising at leastthree loops of continuously wound fiber, which connect the upper endelectrode to the lower end electrode, wherein each of said loopscomprises a first strand and a second strand, wherein a cross-section ofthe first strand is a mirror image of a cross-section of the secondstrand, and wherein the first strand and the second strand each comprisea surface facing the varistor stack, a bursting-protective bandagecomprising a plurality of rings or bands wound of fiber, said bandageradially surrounding the varistor stack and the clamping loops, whereinthe cross sections of the loops are adapted such that shapes of therings or bands are substantially circular, and a surrounding,electrically insulating, outer casing of rubber or other polymericmaterial.
 2. The surge arrester according to claim 1, wherein the loopsare shaped and located so that not only two corners, one on eitherstrand, make contact with the varistor stack.
 3. The surge arresteraccording to claim 1, wherein the cross sections of the loops areadapted to increase a contact surface against the varistor stack.
 4. Thesurge arrester according to claim 1, wherein the cross sections of theloops are adapted to shorten a free span of the rings or bands insidethe loops.
 5. The surge arrester according to claim 1, wherein the crosssections of the loops are adapted to enable the rings or bands to bewound closer to the stack.
 6. The surge arrester according to claim 1,wherein the cross sections of the loops essentially correspond to twomirror-inverted rhombs or rhomboids.
 7. The surge arrester according toclaim 1, wherein the rings or bands comprise aramide fiber or PBO fiberwith an epoxy or vinyl ester matrix.
 8. The surge arrester according toclaim 1, wherein the varistor blocks comprise metal oxide.